The invention relates mainly to a temperature-measuring device for an induction-type cooking appliance and to an appliance comprising such a device.
1. Description of the prior art
There are known devices for measuring the temperature of vessels heated by induction, for example on a cooking hob. Such a cooking hob comprises a plane plate, for example made of glass-ceramic, on which a saucepan can be placed. The temperature of the content of the saucepan is to be measured, for example for the purpose of a servo-control. The measuring device must allow for the fact that the bottom of the saucepans is not necessarily perfectly plane and that saucepans of various diameters can be used. If the saucepan has a concave or convex bottom, the point sensors located at the center risk not coming in contact with a saucepan and therefore giving incorrect measurements.
Furthermore, where an induction-type cooking hob is concerned, even with a flat-bottomed saucepan maximum heating is obtained with a diameter corresponding to half the diameter of the inductor. At this location, the temperature of the bottom of the saucepan is higher than the temperature of its content, for example oil, which it is intended to be able to measure. In contrast, the temperature is lower at the center and edge of the saucepan. It is in the neighborhood of that of the content of the saucepan.
The point sensors arranged on the periphery risk being incapable of measuring the temperature of saucepans of small diameter.
To solve this problem, low-power induction-type cooking hobs have been equipped with aluminum plates put in thermal contact with the temperature sensors. This aluminum plate of high thermal conductivity makes it possible to average the temperature out over a larger surface.
The present invention is based on the discovery that temperature measurements using the device of known type comprising an aluminum plate have been falsified by the heating of the said aluminum plate by the current induced by the inductor. This is especially serious when the measurement of the temperature serves for a servo-control of the latter. In fact, the induction currents heat the aluminum plate and falsify the measurement of the temperature which, in turn, regulates the induction-current power. The result is a completely erratic behavior.
Furthermore, it is absolutely impossible to use aluminum plates for induction-type cooking hobs of normal power or high power. Indeed, at a high power the heating of the aluminum plate risks damaging or destroying the temperature sensors and the inductor. Moreover, at a high power, assuming that the sensor will still be capable of functioning, the measured temperature is mainly a function of the currents induced in the aluminum plate and is influenced only slightly by the cooking temperature.